Abstract
Chemically-powered micromotors offer exciting opportunities in diverse fields, including therapeutic delivery, environmental remediation, and nanoscale manufacturing. However, these nanovehicles require direct addition of high concentration of chemical fuel to the motor solution for their propulsion. We report the efficient vapor-powered propulsion of catalytic micromotors without direct addition of fuel to the micromotor solution. Diffusion of hydrazine vapor from the surrounding atmosphere into the sample solution is instead used to trigger rapid movement of iridium-gold Janus microsphere motors. Such operation creates a new type of remotely-triggered and powered catalytic micro/nanomotors that are responsive to their surrounding environment. This new propulsion mechanism is accompanied by unique phenomena, such as the distinct off-on response to the presence of fuel in the surrounding atmosphere, and spatio-temporal dependence of the motor speed borne out of the concentration gradient evolution within the motor solution. The relationship between the motor speed and the variables affecting the fuel concentration distribution is examined using a theoretical model for hydrazine transport, which is in turn used to explain the observed phenomena. The vapor-powered catalytic micro/nanomotors offer new opportunities in gas sensing, threat detection, and environmental monitoring, and open the door for a new class of environmentally-triggered micromotors.
Highlights
Chemically-powered micromotors offer exciting opportunities in diverse fields, including therapeutic delivery, environmental remediation, and nanoscale manufacturing
A variety of artificial micro/nanomotors propelled by chemical reactions or external stimuli has been developed over the past decade to overcome the challenges of propulsion at low Reynolds numbers and the effects of Brownian motion[11]
Partition of hydrazine from the surrounding atmosphere into the micromotor solution is shown to trigger the efficient movement of Ir-Au microsphere motors without direct addition of the hydrazine fuel to the sample solution
Summary
The Janus micromotors consist of gold particles (1.15 μm diameter) with one hemisphere coated with iridium metal. The effect of different separation distances on the motor speed is illustrated from the 1 second motion track lines of Fig. 3B(a–c), following a five minute exposure to the fuel droplet. We found that motors at equivalent radial distances moved at similar speeds over the range of experimental separation distances considered here These experimental data are in good agreement with our model, where the hydrazine concentration immediately surrounding the small 2.5 mm droplet is assumed to be uniform on the scale of the droplet. The significant spatial dependence of the micromotor speed indicates the negligible effects of convective streams within the sample droplet, such as those due to evaporation If such flows were present and strong, this would lead to mixing and enhancement of the hydrazine transport inside the motor droplet, which could be modeled as an effective diffusivity (at long times). Such ability of micromotors to swim in response to the presence of a remote fuel source offers considerable promise for designing future environmentally-responsive micromachines for a wide range of important future defense and environmental applications
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